In the course of studies on various aspects of reticulum cell neoplasms type B that arise in old BALB/c mice many tumors have been transplanted and are being maintained in mouse passage. B lymphocytes were suspected of being the cell of origin of these tumors; therefore, in a collaborative study the transplanted tumors were tested with immunofluorescence techniques for the presence on cell surfaces of immunoglobulins characteristic of B lymphocytes. Also the cells were tested to determine whether the Ig was secreted into the surrounding medium. Correlations were sought between the type of Ig on the surface of the tumor cells and the site of tumor growth in either the mouse in which the tumor arose or in mice that received transplants of the tumors.